I. Field of Invention
The subject invention relates to laser beam recording and display systems and more particularly to laser beam recording and display systems wherein a closed feedback loop is utilized to accurately determine the position and movement of the writing laser beam.
II. Description of Prior Art
Typical laser beam writing and recording systems are described in U.S. Pat. No. 3,961,334 and copending U.S. application entitled "Method and Apparatus for Recording and Projecting Images" filed Oct. 14, 1977 and having Ser. No. 842,142 the contents of which are incorporated herein by reference. As laser beam recording and display systems become more sophisticated and complex it becomes more desirable and even necessary that the position of the laser writing beam be more accurately controlled. The basic type of laser beam position sensors are absolute and incremental. An absolute sensor indicates the true absolute position of the writing beam at any instant in time whereas an incremental sensor only detects the relative movement of the beam from one resolution element to an adjacent element. Incremental position sensing systems generally have the advantage of requiring simpler optics than absolute systems. They are also more easily expandable. Since the full address of each resolution element is not stored on location as it is in an absolute system, the resolution elements in an incremental system require less space thereby enabling greater resolution to be obtained in the same space.
An incremental, beam position monitoring system is described in the article entitled "Interference Position Monitoring System Employing Ronchi Gratings" by D. P. Jablonswski in the publication Review of Scientific Instruments, Volume 47, no. 1, January 1976 pages 96 through 100. In the system described a distributed interference pattern is utilized to determine the movement of the writing laser beam along the X axis and the Y axis. This is accomplished by utilizing, in addition to the laser writing beam, three additional laser beams having the same frequency but unlike polarization. The operation of the system is such that when the writing laser beam is stationary there is a constant D.C. output. The D.C. output requires that the sensing electronic be DC coupled rather than AC coupled. Sensing electronics that are AC coupled provide a simpler design that is more economical and more immune to laser power shifts and variation of system transmission than that obtainable with DC coupled electronics. Additionally, due to the polarized beams that are utilized, the system requires polarization separators and multiple detectors that add to the cost and complexity of the system. Further, the use of polarized beams for sensing does not enable the use of polarized beams for other functions such as a pointer as is described in the identified application.
Accordingly, one object of this invention is to provide an improved incremental system for sensing the position of a laser writing beam in a laser recording and display system.
Another object of this invention is to provide an improved laser beam recording and display system wherein the writing laser beam sensing means provides a time varying output even when the laser writing beam is stationary.
A further object of this invention is to provide means for sensing the position of a laser beam that enables AC coupled electronics to be used in the position sensing means.
Still another object of this invention is to provide an incremental laser beam position sensing system that is simpler than that heretofore obtainable in the prior art and which is therefore more reliable and economical.